The availability of a culture system which sustains rat liver parenchymal cells provides a unique opportunity to investigate many of the parameters that contribute to differentiation, development and possibly aging. Studies will be directed toward clarifying changes in morphology and function that may accompany the transfer of cells from the host to an artificial medium where the cells may be manipulated to reproduce different stages in their normal as well as abnormal development. The temporal nature of these changes with time in culture and age of the cell will be measured by such parameters as membrane permeability and glycoprotein composition, membrane antigenicity and lectin binding sites, chromosomal protein shifts in relation to cyclic nucleotide content, utilization of folic acid and vitamin B12, and the nature of unique and repeated messenger RNA sequences. Attempts to reproduce in vivo conditions by supplementing the culture media with various hormones, mitogens and serum from young and old animals will also be investigated. Since the response of a cell to its environment may be directly related to the interaction of extrinsic factors with specific membrane receptor sites, the loss of such sites may influence the processes of differentiation, development and aging. Through the use of appropriate media, it may be possible to interconvert various parenchymal cell types (fetal, adult and regenerating) and to demonstrate that these changes are associated with corresponding alterations in membrane composition. Analysis of the membrane by sodium dodecyl sulfate-acrylamide gel electrophoresis following isotope labeling should answer this question in part. Theoretically, it should be possible to revitalize and prolong the life of hepatocytes to survive beyond the six days that they can be maintained in culture at present. Preliminary studies indicate that the cells can be maintained beyond 21 days and possibly longer.